Protein A is a 40,000 Da glycoprotein extracted from the cell wall of various bacteria. Bacteria use PA as a targeting and binding site for tissue attachment. Protein A has a high affinity for the Fc portion of certain immunoglobulin classes and even higher affinity for those immunoglobulins once they have bound antigen. This biochemical property of PA has been used in a large number of applications. These applications of PA reflect a use of the Fc binding properties of the molecule or PA's ability to stimulate humoral immunity in the absence of specific antigen induction (Superantigen applications).